Harold,
I'll give you an educated guess, I believe it's a Athearn reefer. I say that because of the way it's constructed, Athearn reefers have the ice hatches like that and the coupler lids are formed metal, also if you look real close you'll notice that the roof and ends are one piece this is the way Athearn reefers are still made to this day. I don't remember if Athearn stamped any markings on the inside, but I'd lean towards saying the don't. As far as it being 50 years old I don't think you could tell, at least I don't know of a way. It looks in fair shape reguardless of age.

Is it plastic? it looks like it.
50 years ago Athearn was using screw-on lids on the coupler boxes. The trucks came with the worst design of rubber blobs to fill the truck spring holes (most were replaced by after-market springs). They also had 3 large plastic hinge mounts for the operting doors (can't see them in your picture.) The bit where the roof comes down over the doors looks very much like it. This is based on my first Athearn reefer of about 1958. The metal clip-on coupler box lids appeared in the early 60s.

I'm sure the car is an Athearn, but it's age would be very difficult to determine. If it has Athearn sprung trucks, it would be quite old. The thing is those kits are still in production and if the sprung truck is a kaddee truck which has been substituted for the original Athearn, it could be brand new. I doubt that it has much collector value, but it is a nice running car for an operational railroad.